mm/writeback: fix possible divide-by-zero in wb_dirty_limits(), again

(struct dirty_throttle_control *)->thresh is an unsigned long, but is
passed as the u32 divisor argument to div_u64().  On architectures where
unsigned long is 64 bytes, the argument will be implicitly truncated.

Use div64_u64() instead of div_u64() so that the value used in the "is
this a safe division" check is the same as the divisor.

Also, remove redundant cast of the numerator to u64, as that should happen
implicitly.

This would be difficult to exploit in memcg domain, given the ratio-based
arithmetic domain_drity_limits() uses, but is much easier in global
writeback domain with a BDI_CAP_STRICTLIMIT-backing device, using e.g. 
vm.dirty_bytes=(1<<32)*PAGE_SIZE so that dtc->thresh == (1<<32)

Link: https://lkml.kernel.org/r/[email protected]
Fixes: f678959 ("mm/page-writeback.c: fix divide by zero in bdi_dirty_limits()")
Signed-off-by: Zach O'Keefe <[email protected]>
Cc: Maxim Patlasov <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: zokeefe

mm/page-writeback.c

@@ -1638,7 +1638,7 @@ static inline void wb_dirty_limits(struct dirty_throttle_control *dtc)
 	 */
 	dtc->wb_thresh = __wb_calc_thresh(dtc);
 	dtc->wb_bg_thresh = dtc->thresh ?
-		div_u64((u64)dtc->wb_thresh * dtc->bg_thresh, dtc->thresh) : 0;
+		div64_u64(dtc->wb_thresh * dtc->bg_thresh, dtc->thresh) : 0;
 
 	/*
 	 * In order to avoid the stacked BDI deadlock we need